Effect of deep cryogenic treatment and solution treatment on the properties of SAF 2507 super duplex stainless steel

Abstract

Super duplex stainless steel demonstrates remarkable strength and ductility, rendering it an ideal candidate for a broad spectrum of high-end applications as a substitute for austenitic and ferritic stainless steels. The deep cryogenic treatment (DCT) process has been shown to substantially enhance the mechanical properties of austenitic stainless steel. This study investigates the influence of microstructural changes during DCT on the mechanical properties of SAF 2507 super duplex stainless steel forging state through impact and tensile testing. Following the application of the DCT process, as the duration of treatment increases, the high-angle grain boundary structure becomes increasingly prominent. Concurrently, the twin crystal structure within the austenite phase proliferates significantly, resulting in the formation of numerous twin boundaries. Consequently, this leads to a deterioration of the material's plasticity-toughness balance, causing reductions in both impact toughness and elongation after fracture. When the forged SAF 2507 super duplex steel was heated at a rate of 10 °C/min to a temperature range of 1050–1100 °C and subsequently held for 30 min, electron backscatter diffraction scanning characterisation revealed that the austenite-to-ferrite ratio approached 1:1. The microstructure exhibited fine grains and an exceptional strength-to-ductility balance. Specifically, the material demonstrated a tensile strength of 852.68 MPa and an elongation after fracture of 47.78%, indicative of superior mechanical properties.